Radio frequency identification transponder

ABSTRACT

A radio frequency identification transponder including a power supply and a dynamic memory array which stores data. When power from the power supply ceases the data in the dynamic memory array is validly maintained for a predetermined period of time. The dynamic memory array is responsive to an interrogating signal for selectively updating the data. Further claimed is a radio frequency identification transponder wherein a signal processor extracts an identifier from the interrogation signal and is responsive to the identifier and the stored data to determine whether some or all of the identifier is stored in the dynamic memory array. Further claimed is a system wherein a transmitter provides a plurality of temporally spaced interrogating signals which are received by a receiver which incorporates a signal processor that is able to determine the order in which transponders were first in receipt of the interrogating signal. Further claimed is a baggage handling system wherein a transmitter provides a plurality of temporally spaced interrogating signals into an interrogating space through which a conveyor sequentially progresses baggage, a receiver receives transponder response signals which include baggage identity data, a signal processor then extracts the identity data and determines the order in which the baggage has progressed through the interrogating space.

FIELD OF INVENTION

The present invention relates to a radio frequency identification(“RFID”) transponder, and more particularly to an RFID transponder thatare used in orientation independent applications. That is, where atransponder must be operatable in random orientations.

The invention has been developed primarily for interrogating multiplepassive transponders that are attached to objects to be identified bythose respective transponders, and will be described hereinafter withreference to that application. A typical application is theidentification of RFID transponders attached to conveyor fed-luggagewhere the transponder data is used to control the automatic sorting ofthe luggage. However, the invention is not limited to this particularfield of use. For example, various aspects of the invention areapplicable to systems based on active transponders, and to applicationsother than luggage sorting systems.

BACKGROUND

In prior art systems, transponders are read by interrogation fieldswithin Tunnel Reader Programmers (“TRPs”). Typically, the orientationand position of transponders as they enter the TRP are random andunknown. Accordingly, the TRP must switch its interrogation fieldsbetween orthogonal directions so that the transponders can beinterrogated independently of their orientation. U.S. Pat. No. 5,258,766provides an example of such a system.

There are a number of specific issues arising from the practical use ofRFID transponders, in, say, luggage handling situations. These issuesinclude the facts that, for example:

-   -   1. A mechanical means, such as a conveyor, moves luggage (and        thereby transponders) through the TRP.    -   2. Several transponders may be attached to a single item.

Where a mechanical means, such as a conveyor, moves items with attachedtransponders through the TRP the determination of the order of items onthe conveyor is an essential requirement for allowing automatedprocessing of the items. The determination of the order in whichtransponders enter a TRP is advantageous for determining the order ofitems on the conveyor. In prior art systems transponder order isnormally inferred from the order in which they are identified. However,where multiple transponders are present the identification messages fromthese transponders may clash and the transponders may fail to beidentified as they enter the TRP. When messages clash a further timeinterval will be required to correctly identify the transponders. Duringthis time the transponders are moved further into the TRP by theconveyor. It is possible that subsequent transponder or transponders mayenter the TRP before the first transponder is identified. It thenbecomes possible that one or more of the subsequent transponders may beidentified before the first transponder. Consequently, the order ofitems may incorrectly be inferred from the order of transponderidentification.

When the interrogation fields are switched, passive transponders powerdown within a relatively short time, at which point temporary datastored in volatile memory on board the transponder may be lost. Suchdata can include configuration information or temporary data stored inregisters. In prior art systems configuration information or temporarysettings required for transponder operation must be regenerated in thetransponder after each switching of the interrogation field. This datacan be read out of the transponder's memory or may have to betransmitted to the transponder by the TRP. This is undesirable becauseof the time delay involved. Moreover, in some cases, the data may nolonger be available.

The above discussion is not to be taken as an admission of the extent ofthe common general knowledge in the field of the invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome, or at leastsubstantially ameliorate, one or more of the disadvantages of the priorart or at least to provide a useful alternative.

According to a first aspect of the invention there is provided aradio-frequency identification (“RFID”) transponder for use in an RFIDsystem, the transponder including:

a dynamic memory array for storing first data; and

a power supply for powering circuitry associated with the RFIDtransponder, including the dynamic memory array;

wherein the transponder is configured such that, when power ceases to beprovided to the dynamic memory array by the power supply, the first datain the dynamic memory array is validly maintained therein for apredetermined time period; and

wherein the memory is responsive to an interrogating signal from thesystem for selectively updating the first data.

Preferably, the RFID system provides an interrogating signal having anidentifier and the transponder includes a receiver for receiving thesignal. More preferably, the transponder includes a signal processorthat is responsive to:

-   -   a) The receiver for extracting the identifier from the signal;        and    -   b) The identifier and the first data for determining whether the        identifier is stored in the memory array.

In a preferred embodiment, the power supply is configured to providepower by converting an externally applied electromagnetic excitationfield into electrical power.

Preferably also, the first predetermined time period is determined bydischarging of the dynamic memory array through stray leakage pathswithin the dynamic memory array.

Preferably, the RFID transponder further includes timer means forproviding a validity flag in response to an interrogating signal, theflag being “valid” if the interrogating signal is received within asecond time period and “invalid” if the interrogating signal is receivedafter the second time period, the transponder being configured such thatthe data in the dynamic memory array is maintained in a readable stateuntil at least the expiration of the second time period.

In a preferred form, the timer means includes a capacitive cell forstoring a charge generated by current from the power supply, the timermeans being configured such that, once power from the power supplyceases to be supplied, the charge in the capacitive cell discharges at apredetermined rate.

Preferably, the status of the validity flag is based on a voltagegenerated at a predetermined point in the timer means, the voltagedecaying as the capacitive cell discharges. More preferably, the voltageis the output voltage of the capacitive cell as it discharges through aload.

In a preferred embodiment, the capacitive cell is a memory cell. It isparticularly preferable that the memory cell form part of the dynamicmemory array.

According to a second aspect of the invention there is provided an RFIDsystem including a transponder according to the first aspect and atransponder interrogator for providing an interrogation field andreading data from the transponder.

Preferably, the interrogator is configured to sequentially switch anorientation of the interrogation field, and the second time period isselected to exceed the time that the interrogation field is off duringswitching of its orientation.

According to a third aspect of the invention there is provided a methodof using a radio-frequency identification (“RFID”) transponder in anRFID system, the method including the steps of:

storing first data in a dynamic memory array included within thetransponder; and

powering circuitry associated with the RFID transponder, including thedynamic memory array, with a power supply;

validly maintaining the data in the dynamic memory array for apredetermined time period after power ceases to be provided to thedynamic memory array by the power supply; and

being responsive to an interrogating signal from the system forselectively updating the first data.

Preferably, the method includes the additional step of the RFID systemproviding an interrogating signal having an identifier, and thetransponder includes a receiver for receiving the signal. Morepreferably, the transponder includes a signal processor that isresponsive to:

a) The receiver for extracting the identifier from the signal; and

b) The identifier and the first data for determining whether theidentifier is stored in the memory array.

According to a fourth aspect of the invention there is provided aradio-frequency identification (“RFID”) transponder for use with an RFIDinterrogator that provides a plurality of temporally spacedinterrogating signals, wherein the signals include respectiveidentifiers and the transponder includes:

a receiver for receiving the interrogating signals from theinterrogator;

a memory array for storing first data; and

a signal processor being responsive to the receiver for extracting theidentifier from the signal and being responsive to the identifier andthe first data for determining whether some or all of the identifier isstored in the memory array.

If some or all of the identifier is stored in the memory array, it ispreferred that the data stored updates some or all of the first data.

Preferably, the signal processor determines that the identifier isstored in the memory array if the identifier is different to the firstdata. In other embodiments the signal processor determines that theidentifier is stored in the memory array if a predetermined portion ofthe identifier is different to the first data.

Preferably also, the identifier includes data that is selected from oneor more of the following types:

-   -   second data indicative of the interrogator; and    -   third data that is not constant for the respective signals.

More preferably, the third data is indicative of the timing of therespective signal. However, in other embodiments, the third data is apseudo random number or character string. More preferably, the seconddata includes a reference number for the interrogator that is the samefor all the signals provided by the interrogator. Even more preferably,the third data includes a date stamp that varies with the passage oftime. That is, while the reference number will remain constant, the datestamp changes with time to allow greater functionality, as will becomeapparent from the following detailed description.

Preferably, the signal processor determines that the third data isstored in the memory array if the third data is different from the firstdata. More preferably, the signal processor determines that the thirddata is stored in the memory array if the second data is different fromthe first data. That is, in the latter case, the memory array storesinformation—the third data—if it is the first time that the interrogatorhas interrogated the transponder. In some embodiments the second data isstored in the memory whenever the third data is stored in the memoryarray. In other embodiments additional information is derived from theinterrogation signal and selectively stored in the memory array.

Preferably, the signal processor determines that the second data isstored in the memory array if the second data is different to the firstdata. That is, the memory array stores information—the second data—if itis the first time that the interrogator has interrogated thetransponder.

It will be understood that by “interrogated” it is meant that theinterrogation signal is received by the transponder.

In other embodiments the signal processor determines that the identifieris stored in the memory array if the third data is sufficientlydifferent to some or all of the first data. More particularly, inembodiments where the third data includes a date stamp that isindicative of the time of the respective signal, it is preferred thatthat date stamp is compared with the first data to determine whethersufficient time has elapsed since the last store of the third data inthe memory array. If the elapsed time is large, then the more recentthird data is stored in the memory array.

In a preferred form, the transponder includes a transmitter fortransmitting a reply signal to the interrogator in response to aninterrogating signal, wherein the reply signal includes:

-   -   fourth data indicative of the transponder; and    -   fifth data derived from the first data.

Preferably also, the fifth data includes either or both of the seconddata or the third data. More preferably, the fifth data includes thethird data. In some embodiments, all of the respective identifier isstored in the memory array to constitute the first data. A subset ofthese embodiments will include within the reply signal all theinformation contained in the first data. Other embodiments will includeless than all of the information contained within the first data.

The fourth data is preferably a coded string for allowing discriminationbetween other transponders used with the interrogator.

According to a fifth aspect of the invention there is provided a methodof using a radio-frequency identification (“RFID”) transponder with anRFID interrogator that provides a plurality of temporally spacedinterrogating signals, wherein the signals include respectiveidentifiers and the method includes the steps of:

receiving the interrogating signals from the interrogator with areceiver;

storing first data in a memory array; and

being responsive to the receiver for extracting the identifier from thesignal and being responsive to the identifier and the first data fordetermining whether some or all of the identifier is stored in thememory array.

According to a sixth aspect of the invention there is provided aradio-frequency identification (“RFID”) interrogator for interrogating aplurality of RFID transponders, the interrogator including:

a transmitter for providing a plurality of temporally spacedinterrogating signals, wherein the signals include respectiveidentifiers;

a receiver for receiving response signals from the respectivetransponders, the response signals including respective identity datathat is derived from one or more of the interrogator signals; and

a signal processor being responsive to the receiver for extracting theidentity data from the response signals to determine the order in whichthe transponders were first in receipt of an interrogating signal.

Preferably, the interrogator is a TRP and the transponders are attachedto respective articles that are being progressed through the TRP,wherein the signal processor is responsive to the determination of theorder in which the transponders were first in receipt of aninterrogating signal for determining the order in which the articlesprogress through the TRP. More preferably, the articles are aircraftluggage such as suit cases, baggage, boxes or the like.

According to a seventh aspect of the invention there is provided amethod of interrogating a plurality of radio-frequency identification(“RFID”) transponders with an RFID interrogator, the method includingthe steps of:

providing a plurality of temporally spaced interrogating signals to thetransponders, wherein the signals include respective identifiers;

receiving response signals from the respective transponders with areceiver, the response signals including respective identity data thatis derived from one or more of the interrogator signals; and

being responsive to the receiver for extracting the identity data fromthe response signals to determine the order in which the transponderswere first in receipt of an interrogating signal.

According to an eighth aspect of the invention there is provided aplurality of radio-frequency identification (“RFID”) transponders foruse with an RFID interrogator that provides a plurality of temporallyspaced interrogating signals that include respective identifiers, thetransponders each including:

a receiver for receiving one or more of the interrogating signals;

a signal processor being responsive to the receiver for extracting theidentifiers from the one or more signals; and

a transmitter being responsive to the identifiers for transmitting aresponse signal for allowing the interrogator to determine the order inwhich the transponders were first in receipt of an interrogating signal.

Preferably, the identifier includes first data indicative of the time ofthe provision of the interrogating signal and the response signalincludes data derived from the identifier and data unique to therespective transponder. More preferably, the identifier also includessecond data unique to the interrogator and the response signal isderived from the first and the second data.

Preferably also, the transponders each include respective memory arraysfor storing selected data. More preferably, each transponder includes apower supply for supplying power to the transponder, inclusive of thememory array. Even more preferably, each memory array validly maintainsthe selected data for a predetermined time period after power ceases tobe provided to the power supply.

In a preferred form, the memory array is comprised of dynamic memory.More preferably, the memory array is comprised of DRAM. In otherembodiments, however, use is made of RAM.

According to a ninth aspect of the invention there is provided a methodof using a plurality of radio-frequency identification (“RFID”)transponders with an RFID interrogator that provides a plurality oftemporally spaced interrogating signals that include respectiveidentifiers, the method including the steps of:

the transponders receiving one or more of the interrogating signals withrespective receivers;

being responsive to the respective receivers for extracting theidentifiers from the one or more signals; and

transmitting respective response signals in response to the identifiersfor allowing the interrogator to determine the order in which thetransponders were first in receipt of an interrogating signal.

According to a tenth aspect of the invention there is provided a baggagehandling system for baggage that is tagged with respectiveradio-frequency identification (“RFID”) transponders, the systemincluding:

an RFID interrogator that transmits a plurality of temporally spacedinterrogating signals into an interrogating space, wherein the signalsinclude respective identifiers;

a conveyer for sequentially progressing the baggage through theinterrogating space;

a receiver for receiving response signals from the transponders, whereinthe response signals include identity data that is derived from one ormore of the interrogator signals; and

a signal processor being responsive to the receiver for extracting theidentity data from the response signals and thereby determining theorder in which the baggage is progressed through the interrogatingspace.

In some embodiments the system includes a plurality of spaced apartinterrogators and the identifiers include data indicative of theinterrogator that transmitted the respective interrogating signal.

According to an eleventh aspect of the invention there is provided amethod of handling baggage that is tagged with respectiveradio-frequency identification (“RFID”) transponders, the methodincluding the steps of:

transmitting a plurality of temporally spaced interrogating signals intoan interrogating space with an RFID interrogator, wherein the signalsinclude respective identifiers;

sequentially progressing the baggage through the interrogating space;

receiving response signals from the transponders with a receiver,wherein the response signals include identity data that is derived fromone or more of the interrogator signals; and

being responsive to the receiver for extracting the identity data fromthe response signals and thereby determining the order in which thebaggage is progressed through the interrogating space.

Preferably, once a transponder has been “interrogated” and has respondedto that interrogation, it is muted, in that it will not respond tosubsequent interrogation signals. Preferably also, the mute is only inrespect of interrogation signals from the same interrogator. That is, ifthe transponder receives an interrogation signal from anotherinterrogator it will respond. In still further embodiments the firstinterrogated is configured to selectively “un-mute” the interrogatedtransponder by providing in identifier that indicates to the transponderthat it is another interrogator.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art TRP;

FIG. 2 shows waveforms associated with the prior art TRP of FIG. 1;

FIGS. 3( a) and 3(b) show exemplary circuit schematics associated with atransponder memory array according to the present invention;

FIGS. 4( a) and 4(b) show waveforms associated with the exemplarycircuit schematics shown in FIGS. 3( a) and 3(b);

FIG. 5 shows an example circuit for use with a transponder memory arrayaccording to the invention;

FIG. 6 shows an example circuit for use with a transponder memory arrayaccording to the invention;

FIG. 7 is a perspective view of a piece of luggage having attached to ita transponder according to the invention;

FIG. 8 is a block diagram of relevant functional portions of thetransponder according to the invention;

FIG. 9 shows a diagram of a conveyor mounted interrogator according tothe invention with closely spaced items with transponders attached tothe items;

FIG. 10 shows waveforms received by a transponder according to anembodiment of the invention;

FIG. 11 shows an embodiment of the invention having a memory array usingRAM;

FIG. 12 shows a diagram of a baggage handling system according to theinvention including two closely spaced conveyor mounted interrogators;

FIG. 13 shows a block diagram of relevant functional portions of atransponder according to another embodiment of the invention; and

FIG. 14 shows an example circuit of a memory power supply isolator foruse with a transponder according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows a TRP for a prior art transponder system as disclosed byU.S. Pat. No. 5,258,766.

Referring now to FIG. 7, there is illustrated a piece of luggage 700which includes a handle 701 that supports a radio frequencyidentification (“RFID”) transponder 702. In this case, transponder 702is a passive transponder intended to be powered by an interrogationsignal as discussed below. However, it will be appreciated that thetransponder could be active (battery powered) or a hybrid active/passivetransponder. It will also be understood that the use of RFIDtransponders on luggage is only one example of a wide range of uses towhich the technology can be applied.

FIG. 9 shows a diagram of a conveyor mounted TRP 901 with a series ofclosely spaced luggage items 700 on the conveyor such as would beencountered on a baggage handling system in an airport, a train stationor other transport interchange. Each item has one or more RFIDtransponder 702 attached. Each transponder includes a unique characterstring that is transmits, upon interrogation, to allow identification ofthe specific transponder.

To allow automated sorting and to facilitate other physical handlingprocesses involving the items it is necessary to carefully andaccurately determine the order of the items on the conveyor. This, inturn is greatly facilitated if an accurate determination of the order inwhich the transponders enter a TRP can be made.

It has been known to infer the item order on the conveyor from the orderin which the respective transponders are identified by an interrogator.However, with the increasing volumes of items to be handled and theshort time in which to affect that handling, there is constant pressureto increase conveyor speeds and to reduce the spacing between the items.Moreover, the number of possible combinations of arrivals and departuresfrom major terminals is escalating which in turn introduces morecomplexity and more need for increased processing of the items. Theresult of which is to use multiple transponders. However, with prior artdevices this only exponentially increases the risk that theidentification messages from these transponders will clash and thetransponders will fail to be identified as they enter the TRP. Thisfailing of the prior art only compounds the problem, because whenmessages clash a further time interval is required to correctly identifythe transponders. During this further interval the transponders aremoved further into the TRP by the conveyor. This then gives rise to therisk of subsequent transponders entering the TRP before the firsttransponders are identified. It then becomes possible that one or moreof the subsequent transponders will be identified before the firsttransponder. Consequently, with rising conveyor speeds and decreasingdistances between items, the order of the items cannot be reliablyinferred from the order of transponder identification.

As discussed earlier, in the TRP of the present preferred embodiment,the interrogation field is sequentially and periodically switchedbetween orthogonal orientations to ensure that all transponders arepowered regardless of their relative orientations. In one preferredembodiment of the present invention, when the interrogation field isfirst transmitted in an orthogonal orientation, a unique number istransmitted by the TRP as part of the interrogating signal. This numberis representative of the time of the transmission and the particularorthogonal orientation. This number, defined as a time stamp number, isreceived by all transponders that are being powered by the interrogationfield. Those tags that are being powered for the first time store thetime stamp number in a memory. Those tags that already have a time stampnumber stored in memory from an earlier interrogation ignore the newtime stamp number.

It will be appreciated that the direction of the interrogating field isswitched about every 10 ms although in other embodiments differentperiods are used. Moreover, while the preferred embodiment generates anew time stamp at each switching, it is known in other embodiments touse the same time stamp for respective direction for a plurality ofcycles, and in some cases up to ten cycles. This, however, is dependentupon the speed of the conveyor, the density of the items on the conveyorand the length of the TRP.

Returning to the preferred embodiment, each transponder has an on-boardmemory array for storing first data and a signal processor forextracting the time stamp from the interrogating signal. The contents ofthe memory array—the first data—is compared with the extracted timestamp and, under certain conditions, the time stamp is stored in thememory array. More preferably, if the time stamp is stored, itoverwrites some or all of the first data to constitute fresh first data.

That is, the transponders are configured to access the respective timestamps and to selectively store the information in their memory arrays.The decision as to whether the first data is over written is dependentupon whether or not that transponder has previously been interrogated bythe interrogator. If not, then the first data is overwritten.Conversely, if so, then no overwrite occurs. In effect, the first datais representative of the earliest instant the transponder entered theTRP and was powered—that is, interrogated—for the first time by thatinterrogator.

The time stamp number is then included as part of each transpondersreply or response message. When the transponders provide this message itis received by the interrogator to allow the transponders, and thecorresponding items, to be identified and the order of entrance into theTRP to be determined from the time stamp number. The determination oftransponder order is now independent of the rate of identification. Theoperation of the preferred embodiment allows a transponder to traversethe full length of the TRP before it need be identified and the presenceof many transponders inside the TRP is not problematic, as all areidentified without loss of transponder order.

Advantageously, the preferred embodiment allows the velocity with whichtransponders move through the TRP to be much higher than the prior artsystems where transponders must be identified before the nexttransponder enters the TRP.

The apparatus and method of the preferred embodiment is not limited toTRP operation but rather is applicable to any single or multiple axistransponder reader. That is, all these devices are suitable forperiodically transmitting a unique time stamp number so thattransponders newly entered into the interrogation field receives andstores the time stamp. Subsequently the time stamp is included in thetransponder's reply message and the reader is able to infer the earliestinstant that the transponder entered the interrogation field.

It will be appreciated that the response or reply signal that isprovided by the transponder includes information in addition to any datathat is derived from the identifier. This information is usually aunique code or string that allows the transponder itself to beidentified. That is, the response signal, once decoded by theinterrogator, obtains not only data indicative of the particulartransponder, but also feedback as to the timing or order in which thattransponder was first interrogated.

Preferably, the interrogating signal includes an identifier thatincludes, in addition to the time stamp referred to above, a uniquecharacter string or other unique code for allowing identification of theinterrogator. This becomes particularly significant in systems where useis made of a plurality of interrogators. For example, in FIG. 12 thereis illustrated a system where two TRP's—TRP 120 and TRP 121—are closelyspaced on a conveyor. The items and the corresponding transponders exitTRP 120 and enter the next TRP 121 in quick succession. Usingconventional systems with high conveyor speeds it is increasingly commonfor the time interval between exit and entrance to be too short for thetransponder circuits to detect that the TRP has changed. Under theseconditions there is an increasing risk that the transponder will ignorethe time stamp number transmitted by the second TRP. However, theapplication of the invention to this system overcomes those limitationsof the prior art as the transponder will respond to the interrogationsignal from the second interrogator as it will provide an identifierthat is different to that of the first interrogator. More particularly,when the interrogation field is first actuated in a fresh orthogonaldirection a unique number representative of the TRP is transmitted bythe TRP. This number, called the TRP identification number (TRP ID), isreceived by all transponders that are being powered by the interrogationfield. Those tags that are being powered for the first time store theTRP ID in memory. Those tags that already have a TRP ID stored in memoryfrom an earlier interrogation compare this stored TRP ID against thetransmitted TRP ID to determine whether the transponder has entered anew TRP. If the TRP ID's are different the transponder stores the newnumber to memory and proceeds to operate as though they were beingpowered for the first time. In this manner the close spacing of TRP doesnot impair transponder operation.

FIG. 2 shows the interrogation waveforms associated with the prior artTRP shown in FIG. 1. The TRP uses three orthogonal axes along the X, Yand Z directions. It excites these with sine or cosine currents asdescribed in U.S. Pat. No. 5,258,766.

In contrast, FIG. 10 shows the interrogation waveform received by atransponder according to a preferred embodiment of the invention wherethe time stamp number and TRP ID <number1, number2> are transmitted atthe beginning of each orthogonal field. A transponder oriented along oneof the orthogonal axes will periodically experience a power outage whenthe excitation is removed from that axis. Passive transponders powerdown within a relatively short time, at which point data stored involatile memory on board the transponder will be lost if the power downtime is sufficiently long. Such data can include the time stamp number,TRP identification number, configuration information or temporary datastored in registers.

If the configuration information or temporary settings required fortransponder operation are lost then they must be regenerated in thetransponder after each switching of the interrogation field. This datacan be read out of the transponder's memory or may have to betransmitted to the transponder by the TRP. This is undesirable becauseof the time delay involved. Moreover, information such as the time stampwill be no longer available.

To prevent the loss of the transponder's time stamp number, TRP ID,configuration information or other temporary data, the data must bestored in either conventional dynamic memory that is able to hold itsinformation for a period longer than the longest periodic power outage.This memory array will be referred to as “temporary memory”. EEPROM hasthe disadvantage that writing data into memory takes severalmilliseconds. A Random Access Memory RAM or a Dynamic Random AccessMemory DRAM like memory does not suffer from slow write times andconsumes little power. It will be appreciated that RAM will hold itsdata provided the supply voltage is maintained and DRAM will hold itsdata for a short time after the supply voltage has been removed.

FIG. 13 shows a simplified block diagram of specific portions of theRFID transponder's circuits pertinent to the invention. It will beunderstood by those skilled in the art that RFID tags include a numberof functional blocks other than those illustrated. However, these havebeen omitted from the present description for the purposes of clarity,on the basis that those skilled in the field will be aware of the blocksrequired, and how to implement them in an RFID tag. The receiver 130receives the interrogator's interrogating signal which, in thisembodiment, includes an identifier in the form of the time stamp numberand the TRP ID. In other embodiments only one or the other of the timestamp and TRP ID are sent. Moreover, in still further embodimentadditional information is included instead of or in addition to theabove information.

The transponder includes a signal processor 131 that is responsive tothe receiver 130 for extracting the identifier which is supplied inwhole or in part to the on-board memory 132 for storage. The decision asto what information to store, if any, is usually based upon a comparisonof the identifier with the existing contents of the memory. For example,in some instances it will simply be a decision of whether the identifieris different from the stored information, as this will be the case uponthe first interrogation of the transponder as it is usual for the memory132 to contain all zeros or all ones at the time of power up.Accordingly, the decision, in some embodiments, is based upon whetherthe stored date stamp has, in effect, timed out. In other embodiments,however, the decision is based upon a comparison of the received and thestored TRP ID. That is, if the TRP ID is different then the transponderoverwrites the stored data with the freshly received data in recognitionthat the transponder is now being interrogated by another TRP. Infurther embodiments alternative decision making processes are involved.However, the end result of this is that the memory 132 will contain atleast one identifier or at least a sufficient portion of that identifierto allow the transponder to communicate timing details to a TRP wheninterrogated.

The transponder also includes an on-board power supply 133 for providingelectrical power to the transponder. It will be appreciated that thepower supply also supplies power to the memory 132.

Turning to FIG. 8, there is shown a simplified block diagram of specificportions of an embodiment of the invention that makes use of temporarymemory. In particular, an RFID tag 800 is illustrated and includes asubstrate for supporting all the electronic components, some of whichare shown and others of which are omitted for the sake of clarity. Thecomponents of interest are a power supply 801 and a temporary memoryarray in the form of dynamic random access memory (DRAM) 802. The DRAM802 includes a plurality of address cells (described in detail inrelation to FIGS. 3( a) and 5) for holding temporary data used by theRFID transponder and its interrogator. In the preferred embodiment, theDRAM also includes timing means in the form of a modified memory cell803 and associated circuitry (discussed in detail in relation to FIGS.3( b) and 6). Alternative embodiments of the circuit shown in FIG. 8will be discussed later.

FIG. 3( a) shows a schematic of a preferred circuit used for storing onebit of temporary data. It will be appreciated that the memory on-boardthe transponder is comprised or many like circuits to provide thestorage capacity required for the application concerned. Referring backto FIG. 3( a), the memory circuit is a temporary memory circuit wheredata is input at Data In, input data is stored on the storage capacitorby Data Enable and the stored data is available at Data Out. Data isstored on a storage capacitor 301, and once so stored, there is nodischarge path other than leakage currents through M1 and the outputinverter's gate capacitances. These leakage currents are relativelysmall and the storage capacitor voltage will maintain itself for atleast a few seconds. The normal refresh circuits used for maintainingthe data once stored are not shown but are well known and understood tothose in the relevant field.

The data that is stored in the temporary memory will typically bederived from one of two sources. Firstly it can be derived from the TRPwhich transmits the data to the transponder. Secondly it can be derivedinternally; either from the transponder's own memory or generated fromcircuits on the transponder.

FIG. 3( b) shows a schematic of a preferred the circuit used for storingthe Valid DRAM Bit. The chip voltage Vdc is maintained on a storagecapacitor 302 by the refresh circuit formed by M2, M3, a current sourceand an inverter. When the voltage Vdc decreases due to a power outagethe refresh circuit prevents the storage capacitor from dischargingthrough M2. Discharge occurs through the current sink formed by M1 andR. This circuit, when M1 is correctly sized, provides a reliable andstable discharge current. The details of this circuit's operation areprovided in the publication “Switched-Source-Impedance CMOS Circuit ForLow Standby Subthreshold Current Giga-Scale LSI's” pages 1131–1135 ofthe IEEE JOURNAL OF SOLID STATE CIRCUITS. VOL 28 NO 11, NOVEMBER 1993.The discharge current will always be larger than the leakage currents inthe circuit of FIG. 3( a). Accordingly, the circuit shown in 3(b) willalways discharge before the temporary memory circuit and may thereforebe used to indicate the validity of the temporary memory.

FIG. 4( a) shows a representation of the discharge waveform for thetemporary memory circuit. FIG. 4( b) shows a representation of thedischarge waveform for the Valid DRAM Bit circuit. It will be noted thatthe Valid DRAM Bit discharge to the transition point between a logicalone and zero is significantly shorter than the temporary memory circuitbecause of the discharge circuit M1 and R.

FIG. 5 shows a circuit for temporary memory fabricated on a siliconchip, whilst FIG. 6 shows a circuit for a Valid DRAM Bit fabricated on asilicon chip FIG. 11 shows an alternative embodiment of the inventionwhere use is made of temporary memory using RAM, where the electricalpower for the memory 110 is stored on a storage capacitor 111. During apower outage the storage capacitor 111 is isolated from the main powersupply by the series pass transistor 112. A discharge circuit 113 andvoltage level detector 114 connected in parallel with the storagecapacitor 111 is used to limit the maximum time that data remains storedin the memory. The discharge current is small enough to ensure that thememory contents remain valid during normal power outages. If thetransponder remains without power for too long then the storagecapacitor will discharge below the voltage threshold of the leveldetector. When power is reapplied the chip circuits will detect that thestorage capacitor has discharged and recognise that the contents of theRAM are invalid.

FIG. 14 shows a RAM power supply isolation circuit fabricated on asilicon chip for an embodiment of the invention. This provides onespecific example of the implementation of such circuitry and is intendedto be indicative of the layout. It will be appreciated by those skilledin the art that other configurations and layouts are also suitable.

The methods used by the invention are applicable to a number of otheraspects in a transponder system to provide advances over the prior art.Some of these aspects are further described below under that followingheadings.

Mute Chip Bit

Where many transponders are simultaneously present inside a TRP thenumber of transponder identification messages may interfere with theidentification process. It is advantageous to be able to silencetransponder transmissions once the TRP has identified the transponderand completed all necessary dialogue with the transponder. The TRP maytransmit a command, which mutes the transponder preventing furthertransmissions by the transponder. In the prior art systems thesequential and periodic switching of the interrogation field by the TRPresults in transponders periodically powering down.

Unfortunately, when they power down they lose this mute information.This necessitates that the muting information be repetitivelytransmitted for each new orthogonal direction. This reduces the timeavailable for the TRP to receive transponder-identifying messages.

In yet another preferred embodiment of the invention the mutinginformation is stored in the temporary memory, in the form of a “MuteChip Bit”. It is particularly advantageous to store this bit intemporary memory because the bit is not lost as the interrogation fieldis sequentially and periodically switched between the orthogonaldirections. When a transponder powers up it can inspect the Mute ChipBit and determine whether it should be mute.

Configuration Settings

There are a number of transponder functional settings that provideenhanced system performance if they are chosen correctly. These settingscan be advantageously stored in transponder temporary memory where theywill be unaffected by the periodic power outages caused by thesequential switching of the interrogation field. Some examples of theseare provided below to show the utility of using temporary memory.

PRBS Number:

Transponders may use a random number for controlling some functions. Forexample transponders may uses a random time delay between replytransmissions where a random number controls the length of the delay.Alternatively transponders may randomly select a frequency fortransmitting a reply where a random number controls the frequencyselected. An example of such a system is U.S. Pat. No. 5,302,954. Randomnumbers are conveniently generated on chips by Pseudo Random BinarySequence PRBS generators. These are made using long shift registers withfeedback at critical points. For correct operation the binary number inthe shift register must not be lost during the power outages caused bythe sequential switching of the interrogation field. This is achieved ifthe shift register contents are stored in temporary memory, using apreferred embodiment of the present invention.

Chopper Settings:

For applications where there are a large number of transponders present,the number of transponder identification messages may choke thecommunication system. Under these circumstances the proportion oftransponders transmitting must be reduced. One method of achieving thisis to introduce a random time delay between reply transmissions where arandom number controls the length of the delay and the average length ofthe delay is set by the TRP. In this manner only a small portion oftransponders are transmitting at any one time. The larger the averagedelay the smaller the proportion of transponders transmitting. The TRPcan set the average delay length by writing to control bits. These bitsrepresent the “Chopper Setting”. It is advantageous if the ChopperSetting is not lost during the periodic power outages caused by thesequential switching of the interrogation field. This is achieved if theChopper Setting is stored in temporary memory, using a preferredembodiment of the present invention.

Power Mode Control:

For applications where there are a large number of transponders presentand in close proximity, coupling between proximate operatingtransponders may impair transponder operation. Under these circumstancesthe proportion of transponders operating must be reduced so that theaverage distance between operating transponders is increased. Thecoupling between operating transponders is accordingly reduced. Onemethod of achieving this is to introduce a random time delay betweentransponders moving from a low power non-operating state and a normalpower operating state where a random number controls the length of thetime and the average length of the time is set by the TRP. In thismanner only a small portion of transponders are operating at any onetime. The larger the average time the smaller the proportion ofoperating transponders. The TRP can set the average time by writing tocontrol bits. These bits represent the “Power Mode Control” number. Itis advantageous if the Power Mode Control is not lost during theperiodic power outages caused by the sequential switching of theinterrogation field. This is achieved if the Power Mode Control isstored in temporary memory, again, using a preferred embodiment of theinvention.

The above description illustrates that the preferred embodiments of theinvention provide many advantages over the prior art systems. In someembodiments these advantages arise from the provision of a plurality ofRFID transponders for use with an RFID interrogator that provides aplurality of temporally spaced interrogating signals that includerespective identifiers, where the transponders each include:

a receiver for receiving one or more of the interrogating signals;

a signal processor being responsive to the receiver for extracting theidentifiers from the one or more signals; and

a transmitter being responsive to the identifiers for transmitting aresponse signal for allowing the interrogator to determine the order inwhich the transponders were first in receipt of an interrogating signal.

It is inherent in the transponder/TRP system that both the transponderand the TRP have transmitters and receivers for allowing two waycommunication between the devices. However, the preferred embodiments ofthis invention go further and utilise the content and timing of thetransmitted information between the devices in an advantageous andconstructive manner. The preferred embodiments also make use of theon-board processing capability of the transponders, which includes,amongst other components, the signal processor mentioned above, acentral processor and associated memory.

Although the invention has been described with reference to a number ofspecific embodiments and aspects, it will be appreciated that by thoseskilled in the art that the invention can be embodied in many otherforms.

1. A radio-frequency identification (“RFID”) transponder, the RFIDtransponder including: a volatile memory array for storing first data; apower supply for powering circuitry associated with the RFIDtransponder, including the volatile memory array; wherein when powerceases to be provided to the volatile memory array by the power supply,data in the volatile memory array is validly maintained therein for apredetermined time period; wherein the volatile memory array isresponsive to an interrogating signal for selectively updating the firstdata; and wherein the predetermined time period is determined bydischarging of the volatile memory array through stray leakage pathswithin the volatile memory array.
 2. The transponder according to claim1 wherein: the interrogating signal comprises an identifier; and theRFID transponder includes a receiver for receiving the interrogatingsignal.
 3. The transponder according to claim 2, further comprising asignal processor that is responsive to: a) the receiver for extractingthe identifier from the interrogating signal; and b) the identifier andthe first data for determining whether the identifier is stored in thevolatile memory array.
 4. The transponder according to claim 1 whereinthe power supply provides power by converting an externally appliedelectromagnetic excitation field into electrical power.
 5. Thetransponder according to claim 1, further comprising: a timer forproviding a validity flag in response to the interrogating signal;wherein to flag is “valid” if the interrogating signal is receivedwithin a second time period; wherein the flag is “invalid” if theinterrogating signal is received after the second time period; andwherein the data in the volatile memory array is maintained in areadable state until at least the expiration of the second time period.6. The transponder according to claim 5 wherein the timer comprises acapacitive cell for storing a charge generated by current from the powersupply, wherein once power from the power supply ceases to be supplied,the charge in the capacitive cell discharges at a predetermined rate. 7.The transponder according to claim 5 wherein a status of the validityflag is based on a voltage generated at a predetermined point in thetimer, the voltage decaying as the capacitive cell discharges.
 8. Thetransponder according to claim 7 wherein the voltage is an outputvoltage of the capacitive cell as it discharges through a load.
 9. Thetransponder according to claim 8 wherein the capacitive cell is a memorycell.
 10. The transponder according to claim 9 wherein the memory cellforms part of the volatile memory array.
 11. An RFID system comprising:an RFID transponder according to claim 1; and a transponder interrogatorfor providing an interrogation field and reading data from the RFIDtransponder.
 12. The system according to claim 11 wherein theinterrogator sequentially switches an orientation of the interrogationfield.
 13. A method of using a radio-frequency identification (“RFID”)transponder, the method including the steps of: storing data in avolatile memory array included within the RFID transponder; poweringcircuitry associated with the RFID transponder, including the volatilememory array, with a power supply; validly maintaining the data in thevolatile memory array for a predetermined time period after power ceasesto be provided to the volatile memory array by the power supply; beingresponsive to an interrogating signal for selectively updating the firstdata; and wherein the predetermined time period is determined bydischarging of the volatile memory array through stray leakage pathswithin the volatile memory array.
 14. The method according to claim 13including the additional step of receiving the interrogating signal, theinterrogating signal comprising an identifier, where in the RFIDtransponder includes a receiver for receiving the signal.
 15. The methodaccording to claim 14 wherein the RFID transponder includes a signalprocessor responsive to: a) the receiver fix extracting the identifierfrom the interrogating signal; and b) the identifier and the first datafor determining whether the identifier is stored in the memory array.